1. Field of the Invention
This invention relates to a method for the production of thermoplastic tubes, the interior surface of each of which is very smooth, so that they are useful as a piping material for handling ultra-pure water.
2. Description of the Prior Art
A large amount of pure or ultra-pure water is used in the production process of semiconductor device (LSI) chips, in the semiconductor device manufacturing industries. Patterns of the size of approximately 2 .mu.m are formed on the LSI chips, and thus impurities of 0.2 .mu.m or more; which may be contained in the ultra-pure water used for the developing process of photoresists on the LSI chips, the etching process of LSI chips and the elimination process of the photoresists from the LSI chips; cause breaking of LSI circuits thereby reducing the quality of the LSI chips and decreasing the yield. Particularly, organic matters, phosphorus compounds, etc., contained in the cell of bacteria, cause deterioration of the LSI chips. If pipes for transporting the ultra-pure water have irregularities on the interior surface, the water stagnated in the area of the irregulatories results in the growth of the bacteria.
Such pipes are usually made of rigid poly-vinylchloride, on the interior surface of which numberless irregularities having a height of approximately 10 .mu.m exist and bacteria increase therein, thereby requiring periodical cleaning thereof.
U.S. Pat. No. 3,644,696 discloses an apparatus for smoothing the interior surface of a plastic hose, wherein a heating unit is positioned inside of the plastic hose obtained by extrusion molding and an induction coil, which is concentric with the heating unit and is positioned around the outside of the hose. The interior surface of the hose is heated by induction heating based on the heating unit and the induction coil to permit the surface tension of the plastics to smooth out the irregularities.
However, such a heating treatment of the interior surface of the house merely allows acute angle portions in the irregularities to be rounded, and it cannot eliminate the irregularities themselves. Deformation of the irregularities due to the heating treatment is restored in the succeeding cooling step. Therefore, the smoothness desired for pipes transporting ultra-pure water therethrough cannot be obtained from such a heating treatment alone.
If the interior surface of the hose is heated to a high temperature which is sufficient to eliminate the irregularities, the plastics (i.e., poly-vinylchloride) decomposes resulting in a drift therein, so that hoses having a predetermined dimension cannot be produced continuously. Moreover, since the interior surface is at an exceedingly high temperature, it is insufficiently cooled in the succeeding cooling step and/or a temperature gradient in the direction of the thickness of the hose is great enough to produce residual strain, resulting in unstable dimensions of the hose.